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                <ol class="chapter"><li class="chapter-item expanded affix "><a href="../翻译说明.html">翻译说明</a></li><li class="chapter-item expanded affix "><a href="../introduction.html">介绍</a></li><li class="chapter-item expanded "><a href="../notation.html"><strong aria-hidden="true">1.</strong> 表义符</a></li><li class="chapter-item expanded "><a href="../lexical-structure.html"><strong aria-hidden="true">2.</strong> 词法结构</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../input-format.html"><strong aria-hidden="true">2.1.</strong> 输入格式</a></li><li class="chapter-item expanded "><a href="../keywords.html"><strong aria-hidden="true">2.2.</strong> 关键字</a></li><li class="chapter-item expanded "><a href="../identifiers.html"><strong aria-hidden="true">2.3.</strong> 标识符</a></li><li class="chapter-item expanded "><a href="../comments.html"><strong aria-hidden="true">2.4.</strong> 注释</a></li><li class="chapter-item expanded "><a href="../whitespace.html"><strong aria-hidden="true">2.5.</strong> 空白符</a></li><li class="chapter-item expanded "><a href="../tokens.html"><strong aria-hidden="true">2.6.</strong> token</a></li><li class="chapter-item expanded "><a href="../paths.html"><strong aria-hidden="true">2.7.</strong> 路径</a></li></ol></li><li class="chapter-item expanded "><a href="../macros.html"><strong aria-hidden="true">3.</strong> 宏</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../macros-by-example.html"><strong aria-hidden="true">3.1.</strong> 声明宏</a></li><li class="chapter-item expanded "><a href="../procedural-macros.html"><strong aria-hidden="true">3.2.</strong> 过程宏</a></li></ol></li><li class="chapter-item expanded "><a href="../crates-and-source-files.html"><strong aria-hidden="true">4.</strong> crate 和源文件</a></li><li class="chapter-item expanded "><a href="../conditional-compilation.html"><strong aria-hidden="true">5.</strong> 条件编译</a></li><li class="chapter-item expanded "><a href="../items.html"><strong aria-hidden="true">6.</strong> 程序项</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../items/modules.html"><strong aria-hidden="true">6.1.</strong> 模块</a></li><li class="chapter-item expanded "><a href="../items/extern-crates.html"><strong aria-hidden="true">6.2.</strong> 外部crate</a></li><li class="chapter-item expanded "><a href="../items/use-declarations.html" class="active"><strong aria-hidden="true">6.3.</strong> use声明</a></li><li class="chapter-item expanded "><a href="../items/functions.html"><strong aria-hidden="true">6.4.</strong> 函数</a></li><li class="chapter-item expanded "><a href="../items/type-aliases.html"><strong aria-hidden="true">6.5.</strong> 类型别名</a></li><li class="chapter-item expanded "><a href="../items/structs.html"><strong aria-hidden="true">6.6.</strong> 结构体</a></li><li class="chapter-item expanded "><a href="../items/enumerations.html"><strong aria-hidden="true">6.7.</strong> 枚举</a></li><li class="chapter-item expanded "><a href="../items/unions.html"><strong aria-hidden="true">6.8.</strong> 联合体</a></li><li class="chapter-item expanded "><a href="../items/constant-items.html"><strong aria-hidden="true">6.9.</strong> 常量项</a></li><li class="chapter-item expanded "><a href="../items/static-items.html"><strong aria-hidden="true">6.10.</strong> 静态项</a></li><li class="chapter-item expanded "><a href="../items/traits.html"><strong aria-hidden="true">6.11.</strong> trait</a></li><li class="chapter-item expanded "><a href="../items/implementations.html"><strong aria-hidden="true">6.12.</strong> 实现</a></li><li class="chapter-item expanded "><a href="../items/external-blocks.html"><strong aria-hidden="true">6.13.</strong> 外部块</a></li><li class="chapter-item expanded "><a href="../items/generics.html"><strong aria-hidden="true">6.14.</strong> 类型参数和生存期参数</a></li><li class="chapter-item expanded "><a href="../items/associated-items.html"><strong aria-hidden="true">6.15.</strong> 关联程序项</a></li><li class="chapter-item expanded "><a href="../visibility-and-privacy.html"><strong aria-hidden="true">6.16.</strong> 可见性与隐私权</a></li></ol></li><li class="chapter-item expanded "><a href="../attributes.html"><strong aria-hidden="true">7.</strong> 属性</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../attributes/testing.html"><strong aria-hidden="true">7.1.</strong> 测试</a></li><li class="chapter-item expanded "><a href="../attributes/derive.html"><strong aria-hidden="true">7.2.</strong> 派生</a></li><li class="chapter-item expanded "><a href="../attributes/diagnostics.html"><strong aria-hidden="true">7.3.</strong> 诊断</a></li><li class="chapter-item expanded "><a href="../attributes/codegen.html"><strong aria-hidden="true">7.4.</strong> 代码生成</a></li><li class="chapter-item expanded "><a href="../attributes/limits.html"><strong aria-hidden="true">7.5.</strong> 极限设置</a></li><li class="chapter-item expanded "><a href="../attributes/type_system.html"><strong aria-hidden="true">7.6.</strong> 类型系统</a></li></ol></li><li class="chapter-item expanded "><a href="../statements-and-expressions.html"><strong aria-hidden="true">8.</strong> 语句和表达式</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../statements.html"><strong aria-hidden="true">8.1.</strong> 语句</a></li><li class="chapter-item expanded "><a href="../expressions.html"><strong aria-hidden="true">8.2.</strong> 表达式</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../expressions/literal-expr.html"><strong aria-hidden="true">8.2.1.</strong> 字面量表达式</a></li><li class="chapter-item expanded "><a href="../expressions/path-expr.html"><strong aria-hidden="true">8.2.2.</strong> 路径表达式</a></li><li class="chapter-item expanded "><a href="../expressions/block-expr.html"><strong aria-hidden="true">8.2.3.</strong> 块表达式</a></li><li class="chapter-item expanded "><a href="../expressions/operator-expr.html"><strong aria-hidden="true">8.2.4.</strong> 运算符表达式</a></li><li class="chapter-item expanded "><a href="../expressions/grouped-expr.html"><strong aria-hidden="true">8.2.5.</strong> 分组表达式</a></li><li class="chapter-item expanded "><a href="../expressions/array-expr.html"><strong aria-hidden="true">8.2.6.</strong> 数组和索引表达式</a></li><li class="chapter-item expanded "><a href="../expressions/tuple-expr.html"><strong aria-hidden="true">8.2.7.</strong> 元组和索引表达式</a></li><li class="chapter-item expanded "><a href="../expressions/struct-expr.html"><strong aria-hidden="true">8.2.8.</strong> 结构体表达式</a></li><li class="chapter-item expanded "><a href="../expressions/enum-variant-expr.html"><strong aria-hidden="true">8.2.9.</strong> 枚举变体表达式</a></li><li class="chapter-item expanded "><a href="../expressions/call-expr.html"><strong aria-hidden="true">8.2.10.</strong> 调用表达式</a></li><li class="chapter-item expanded "><a href="../expressions/method-call-expr.html"><strong aria-hidden="true">8.2.11.</strong> 方法调用表达式</a></li><li class="chapter-item expanded "><a href="../expressions/field-expr.html"><strong aria-hidden="true">8.2.12.</strong> 字段访问表达式</a></li><li class="chapter-item expanded "><a href="../expressions/closure-expr.html"><strong aria-hidden="true">8.2.13.</strong> 闭包表达式</a></li><li class="chapter-item expanded "><a href="../expressions/loop-expr.html"><strong aria-hidden="true">8.2.14.</strong> 循环表达式</a></li><li class="chapter-item expanded "><a href="../expressions/range-expr.html"><strong aria-hidden="true">8.2.15.</strong> 区间表达式</a></li><li class="chapter-item expanded "><a href="../expressions/if-expr.html"><strong aria-hidden="true">8.2.16.</strong> if 和 if let 表达式</a></li><li class="chapter-item expanded "><a href="../expressions/match-expr.html"><strong aria-hidden="true">8.2.17.</strong> 匹配表达式</a></li><li class="chapter-item expanded "><a href="../expressions/return-expr.html"><strong aria-hidden="true">8.2.18.</strong> 返回表达式</a></li><li class="chapter-item expanded "><a href="../expressions/await-expr.html"><strong aria-hidden="true">8.2.19.</strong> 等待(await)表达式</a></li></ol></li></ol></li><li class="chapter-item expanded "><a href="../patterns.html"><strong aria-hidden="true">9.</strong> 模式</a></li><li class="chapter-item expanded "><a href="../type-system.html"><strong aria-hidden="true">10.</strong> 类型系统</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../types.html"><strong aria-hidden="true">10.1.</strong> 类型</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../types/boolean.html"><strong aria-hidden="true">10.1.1.</strong> 布尔型</a></li><li class="chapter-item expanded "><a href="../types/numeric.html"><strong aria-hidden="true">10.1.2.</strong> 数字型</a></li><li class="chapter-item expanded "><a href="../types/textual.html"><strong aria-hidden="true">10.1.3.</strong> 字符型</a></li><li class="chapter-item expanded "><a href="../types/never.html"><strong aria-hidden="true">10.1.4.</strong> never类型</a></li><li class="chapter-item expanded "><a href="../types/tuple.html"><strong aria-hidden="true">10.1.5.</strong> 元组</a></li><li class="chapter-item expanded "><a href="../types/array.html"><strong aria-hidden="true">10.1.6.</strong> 数组</a></li><li class="chapter-item expanded "><a href="../types/slice.html"><strong aria-hidden="true">10.1.7.</strong> 切片</a></li><li class="chapter-item expanded "><a href="../types/struct.html"><strong aria-hidden="true">10.1.8.</strong> 结构体</a></li><li class="chapter-item expanded "><a href="../types/enum.html"><strong aria-hidden="true">10.1.9.</strong> 枚举</a></li><li class="chapter-item expanded "><a href="../types/union.html"><strong aria-hidden="true">10.1.10.</strong> 联合体</a></li><li class="chapter-item expanded "><a href="../types/function-item.html"><strong aria-hidden="true">10.1.11.</strong> 函数项类型</a></li><li class="chapter-item expanded "><a href="../types/closure.html"><strong aria-hidden="true">10.1.12.</strong> 闭包</a></li><li class="chapter-item expanded "><a href="../types/pointer.html"><strong aria-hidden="true">10.1.13.</strong> 指针型</a></li><li class="chapter-item expanded "><a href="../types/function-pointer.html"><strong aria-hidden="true">10.1.14.</strong> 函数指针</a></li><li class="chapter-item expanded "><a href="../types/trait-object.html"><strong aria-hidden="true">10.1.15.</strong> trait对象</a></li><li class="chapter-item expanded "><a href="../types/impl-trait.html"><strong aria-hidden="true">10.1.16.</strong> 实现trait</a></li><li class="chapter-item expanded "><a href="../types/parameters.html"><strong aria-hidden="true">10.1.17.</strong> 类型参数</a></li><li class="chapter-item expanded "><a href="../types/inferred.html"><strong aria-hidden="true">10.1.18.</strong> 推断型</a></li></ol></li><li class="chapter-item expanded "><a href="../dynamically-sized-types.html"><strong aria-hidden="true">10.2.</strong> 动态尺寸类型(DST)</a></li><li class="chapter-item expanded "><a href="../type-layout.html"><strong aria-hidden="true">10.3.</strong> 类型布局 </a></li><li class="chapter-item expanded "><a href="../interior-mutability.html"><strong aria-hidden="true">10.4.</strong> 内部可变性</a></li><li class="chapter-item expanded "><a href="../subtyping.html"><strong aria-hidden="true">10.5.</strong> 子类型和型变</a></li><li class="chapter-item expanded "><a href="../trait-bounds.html"><strong aria-hidden="true">10.6.</strong> trait约束及其生存期约束</a></li><li class="chapter-item expanded "><a href="../type-coercions.html"><strong aria-hidden="true">10.7.</strong> 类型自动强转</a></li><li class="chapter-item expanded "><a href="../destructors.html"><strong aria-hidden="true">10.8.</strong> 析构函数</a></li><li class="chapter-item expanded "><a href="../lifetime-elision.html"><strong aria-hidden="true">10.9.</strong> 生存期省略</a></li></ol></li><li class="chapter-item expanded "><a href="../special-types-and-traits.html"><strong aria-hidden="true">11.</strong> 特殊类型和 trait</a></li><li class="chapter-item expanded "><a href="../memory-model.html"><strong aria-hidden="true">12.</strong> 内存模型</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../memory-allocation-and-lifetime.html"><strong aria-hidden="true">12.1.</strong> 内存分配和其生存期</a></li><li class="chapter-item expanded "><a href="../memory-ownership.html"><strong aria-hidden="true">12.2.</strong> 内存所有权</a></li><li class="chapter-item expanded "><a href="../variables.html"><strong aria-hidden="true">12.3.</strong> 变量</a></li></ol></li><li class="chapter-item expanded "><a href="../linkage.html"><strong aria-hidden="true">13.</strong> 链接(linkage)</a></li><li class="chapter-item expanded "><a href="../unsafety.html"><strong aria-hidden="true">14.</strong> 非安全性</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../unsafe-functions.html"><strong aria-hidden="true">14.1.</strong> 非安全函数</a></li><li class="chapter-item expanded "><a href="../unsafe-blocks.html"><strong aria-hidden="true">14.2.</strong> 非安全代码块</a></li><li class="chapter-item expanded "><a href="../behavior-considered-undefined.html"><strong aria-hidden="true">14.3.</strong> 未定义行为</a></li><li class="chapter-item expanded "><a href="../behavior-not-considered-unsafe.html"><strong aria-hidden="true">14.4.</strong> 不被认为是非安全的行为</a></li></ol></li><li class="chapter-item expanded "><a href="../const_eval.html"><strong aria-hidden="true">15.</strong> 常量求值</a></li><li class="chapter-item expanded "><a href="../abi.html"><strong aria-hidden="true">16.</strong> ABI</a></li><li class="chapter-item expanded "><a href="../runtime.html"><strong aria-hidden="true">17.</strong> Rust运行时</a></li><li class="chapter-item expanded "><a href="../appendices.html"><strong aria-hidden="true">18.</strong> 附录</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../macro-ambiguity.html"><strong aria-hidden="true">18.1.</strong> 宏定义规范</a></li><li class="chapter-item expanded "><a href="../influences.html"><strong aria-hidden="true">18.2.</strong> 影响来源</a></li><li class="chapter-item expanded "><a href="../glossary.html"><strong aria-hidden="true">18.3.</strong> 术语表</a></li><li class="chapter-item expanded "><a href="../本书术语翻译对照表.html"><strong aria-hidden="true">18.4.</strong> 本书术语翻译对照表</a></li></ol></li></ol>
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                        <h1><a class="header" href="#use-declarations" id="use-declarations">Use declarations</a></h1>
<h1><a class="header" href="#use声明" id="use声明">Use声明</a></h1>
<blockquote>
<p><a href="https://github.com/rust-lang/reference/blob/master/src/items/use-declarations.md">use-declarations.md</a><br />
commit: da3098c49b2f62c878ca9a36099d7dffb4bd9474 <br />
本章译文最后维护日期：2020-11-8</p>
</blockquote>
<blockquote>
<p><strong><sup>句法:</sup></strong><br />
<em>UseDeclaration</em> :<br />
   <code>use</code> <em>UseTree</em> <code>;</code></p>
<p><em>UseTree</em> :<br />
      (<a href="../paths.html#simple-paths"><em>SimplePath</em></a><sup>?</sup> <code>::</code>)<sup>?</sup> <code>*</code><br />
   | (<a href="../paths.html#simple-paths"><em>SimplePath</em></a><sup>?</sup> <code>::</code>)<sup>?</sup> <code>{</code> (<em>UseTree</em> ( <code>,</code>  <em>UseTree</em> )<sup>*</sup> <code>,</code><sup>?</sup>)<sup>?</sup> <code>}</code><br />
   | <a href="../paths.html#simple-paths"><em>SimplePath</em></a> ( <code>as</code> ( <a href="../identifiers.html">IDENTIFIER</a> | <code>_</code> ) )<sup>?</sup></p>
</blockquote>
<p><em>use声明</em>用来创建一个或多个与程序项<a href="../paths.html">路径</a>同义的本地名称绑定。通常使用 <code>use</code>声明来缩短引用模块所需的路径。这些声明可以出现在<a href="modules.html">模块</a>和<a href="../expressions/block-expr.html">块</a>中，但通常在作用域顶部。</p>
<p>use声明支持多种便捷方法:</p>
<ul>
<li>使用带有花括号的 glob-like(<code>::</code>) 句法 <code>use a::b::{c, d, e::f, g::h::i};</code> 来同时绑定一个系列有共同前缀的路径。</li>
<li>使用关键字 <code>self</code>，例如 <code>use a::b::{self, c, d::e};</code>，来同时绑定一系列有共同前缀和共同父模块的路径。</li>
<li>使用句法 <code>use p::q::r as x;</code> 将编译目标名称重新绑定为新的本地名称。这种也可以和上面两种方法一起使用：<code>use a::b::{self as ab, c as abc}</code>。</li>
<li>使用星号通配符句法 <code>use a::b::*;</code> 来绑定与给定前缀匹配的所有路径。</li>
<li>将前面的方法嵌套重复使用，例如 <code>use a::b::{self as ab, c, d::{*, e::f}};</code>。</li>
</ul>
<p><code>use</code>声明的一个示例：</p>
<pre><pre class="playground"><code class="language-rust">use std::option::Option::{Some, None};
use std::collections::hash_map::{self, HashMap};

fn foo&lt;T&gt;(_: T){}
fn bar(map1: HashMap&lt;String, usize&gt;, map2: hash_map::HashMap&lt;String, usize&gt;){}

fn main() {
    // 等价于 'foo(vec![std::option::Option::Some(1.0f64), std::option::Option::None]);'
    foo(vec![Some(1.0f64), None]);

    // `hash_map` 和 `HashMap` 在当前作用域内都有效.
    let map1 = HashMap::new();
    let map2 = hash_map::HashMap::new();
    bar(map1, map2);
}
</code></pre></pre>
<h2><a class="header" href="#use-visibility" id="use-visibility"><code>use</code> Visibility</a></h2>
<h2><a class="header" href="#use可见性" id="use可见性"><code>use</code>可见性</a></h2>
<p>与其他程序项一样，默认情况下，<code>use</code>声明对包含它的模块来说是私有的。同样的，如果使用关键字 <code>pub</code> 进行限定，<code>use</code>声明也可以是公有的。<code>use</code>声明可用于*重导出(re-expor)*名称。因此，公有的 <code>use</code>声明可以将某些公有名称重定向到不同的目标定义中：甚至是位于不同模块内具有私有可见性的规范路径定义中。如果这样的重定向序列形成一个循环或不能明确地解析，则会导致编译期错误。</p>
<p>重导出的一个示例：</p>
<pre><pre class="playground"><code class="language-rust">mod quux {
    pub use self::foo::{bar, baz};
    pub mod foo {
        pub fn bar() {}
        pub fn baz() {}
    }
}

fn main() {
    quux::bar();
    quux::baz();
}
</code></pre></pre>
<p>在本例中，模块 <code>quux</code> 重导出了在模块 <code>foo</code> 中定义的两个公共名称。</p>
<h2><a class="header" href="#use-paths" id="use-paths"><code>use</code> Paths</a></h2>
<h2><a class="header" href="#use路径" id="use路径"><code>use</code>路径</a></h2>
<blockquote>
<p><strong>注意</strong>：本章节内容还不完整。</p>
</blockquote>
<p>一些正常和不正常的使用 <code>use</code>程序项的例子：</p>
<!-- 注意: 这个例子在 2015 版或 2018 版都能正常工作。 -->
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused_imports)]
</span>use std::path::{self, Path, PathBuf};  // good: std 是一个 crate 名称
use crate::foo::baz::foobaz;    // good: foo 在当前 crate 的第一层

mod foo {

    pub mod example {
        pub mod iter {}
    }

    use crate::foo::example::iter; // good: foo 在当前 crate 的第一层
//  use example::iter;      // 在 2015 版里不行，2015 版里相对路径必须以 `self` 开头; 2018 版这样写没问题
    use self::baz::foobaz;  // good: `self` 指的是 'foo' 模块
    use crate::foo::bar::foobar;   // good: foo 在当前 crate 的第一层

    pub mod bar {
        pub fn foobar() { }
    }

    pub mod baz {
        use super::bar::foobar; // good: `super` 指的是 'foo' 模块
        pub fn foobaz() { }
    }
}

fn main() {}
</code></pre></pre>
<p>：</p>
<blockquote>
<p><strong>版本差异</strong>: 在 2015 版中，<code>use</code>路径也允许访问 crate 根模块中的程序项。继续使用上面的例子，那以下 <code>use</code>路径的用法在 2015 版中有效，在 2018 版中就无效了:</p>
<pre><pre class="playground"><code class="language-rust edition2015"><span class="boring">mod foo {
</span><span class="boring">    pub mod example { pub mod iter {} }
</span><span class="boring">    pub mod baz { pub fn foobaz() {} }
</span><span class="boring">}
</span>use foo::example::iter;
use ::foo::baz::foobaz;
<span class="boring">fn main() {}
</span></code></pre></pre>
<p>2015 版不允许用 use声明来引用<a href="extern-crates.html#extern-prelude">外部预导入包</a>里的 crate。因此，在2015 版中仍然需要使用 <a href="extern-crates.html"><code>extern crate</code></a>声明，以便在 use声明中去引用外部 crate。从 2018 版开始，use声明可以像 <code>extern crate</code> 一样指定外部 crate 依赖关系。</p>
<p>在 2018 版中，如果本地程序项与外部的 crate 名称相同，那么使用该 crate 名称需要一个前导的 <code>::</code> 来明确地选择该 crate 名称。这种做法是为了与未来可能发生的更改保持兼容。<!-- uniform_paths future-proofing --></p>
<pre><pre class="playground"><code class="language-rust edition2018">// use std::fs; // 错误, 这样有歧义.
use ::std::fs;  // 从`std` crate 里导入, 不是下面这个 mod.
use self::std::fs as self_fs;  // 从下面这个 mod 导入.

mod std {
    pub mod fs {}
}
<span class="boring">fn main() {}
</span></code></pre></pre>
</blockquote>
<h2><a class="header" href="#underscore-imports" id="underscore-imports">Underscore Imports</a></h2>
<h2><a class="header" href="#下划线导入" id="下划线导入">下划线导入</a></h2>
<p>通过使用形如为 <code>use path as _</code> 的带下划线的 use声明，可以在不绑定名称的情况下导入程序项。这对于导入一个 trait 特别有用，这样就可以在不导入 trait 的 symbol 的情况下使用这个 trait 的方法，例如，如果 trait 的 symbol 可能与另一个 symbol 冲突。再一个例子是链接外部的 crate 而不导入其名称。</p>
<p>使用星号全局导入(Asterisk glob imports,即 <code>::*</code>)句法将以 <code>_</code> 的形式导入能匹配到的所有程序项，但这些程序项在当前作用域中将处于不可命名的状态。</p>
<pre><pre class="playground"><code class="language-rust">mod foo {
    pub trait Zoo {
        fn zoo(&amp;self) {}
    }

    impl&lt;T&gt; Zoo for T {}
}

use self::foo::Zoo as _;
struct Zoo;  // 下划线形式的导入就是为了避免和这个程序项在名字上起冲突

fn main() {
    let z = Zoo;
    z.zoo();
}
</code></pre></pre>
<p>在宏扩展之后会创建一些惟一的、不可命名的 symbols，这样宏就可以安全地扩展出(emit)对 <code>_</code> 导入的多个引用。例如，下面代码不应该产生错误:</p>
<pre><pre class="playground"><code class="language-rust">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>macro_rules! m {
    ($item: item) =&gt; { $item $item }
}

m!(use std as _;);
// 这会扩展出:
// use std as _;
// use std as _;
<span class="boring">}
</span></code></pre></pre>
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